Many receivers and receiver decoders have been used to receive a modulated signal so that the receiver output may control some load remote from the transmitter. In remotely controlled devices, radio controlled garage door openers, as an example, there may be an insufficient number of carrier frequencies and modulation frequencies with which to provide sufficient codes so that a neighborhood having many radio controlled garage door operators will incur false operation. There may be interference between two closely adjacent receivers wherein a strong signal from a closely positioned transmitter or the radiation from one of the receivers could energize a relay in a receiver even when the carrier and modulation frequencies in the transmitter and receiver are not the same. One solution was to provide two modulation frequencies both modulating the carrier at the same time and then the receiver had to be on the same code of three different frequencies namely, the carrier and the two modulation frequencies in order to respond to such transmitted signal. Due to the economic limitations, the number of carrier frequencies available was not large. Also the number of modulation frequencies available which were not integral multiples, that is, harmonics of one another, was also not large so that the number of possible codes might be limited to two or three hundred, for example. Often this was insufficient to prevent unwanted actuation of the incorrect receiver and thus opening of a garage door other than the one intended.
One possible solution was to utilize a carrier wave turned on and off by a digital signal. First and second digital signals of two to one ratio of frequencies in phase agreement were utilized and were combined to provide either a short pulse or a long pulse. This gave a pulse train of 9 or 10 pulses of varying pulse widths to turn on and off a carrier wave. A synchronization period of the absence of pulses indicated the end of the message. This message was repeated continuously by the transmitter. If the receiver was on the same code, that is, the same carrier frequency and the same sequence of short and long pulses, then the receiver would have an output and the remotely controlled load would be actuated.
These prior attempts at a receiver decoder had the deficiencies of an insufficient number of codes to be transmitted so as to avoid improper actuation of the receiver. There have been reports that the digital control system has been false triggered by noise such as a teletype signal which also has short and long pulses in its signal. Also such prior digital control system had a lack of a preamble pulse pattern to assure exclusivity of the transmitted and received signal. Also such prior digital system could be easily false triggered by a noise pulse so that the entire train of pulses was dumped at the wrong time.